Air blown fiber systems are now available as an alternative to conventional fiber optic cabling systems. These systems are designed to save space and time, and to facilitate system design changes. In an air blown fiber installation, the optical fiber medium is propelled through a pre-installed cable tube by a viscous air flow. Using this technique, the optical fiber medium is mechanically “pushed” into the duct concurrently with a stream of air, with the net force distributed along the fiber length rather than pulled from one end. Advantages in addition to simplicity and flexibility are that fiber breaks or excessive stress are minimized.
In a typical installation, the cable installation route comprises a “highway” of individual inner sub-ducts bundled inside a protective outer duct. A variety of duct styles is used, adapted specifically for use in plenum, riser, general purpose and outdoor applications. The inner sub-ducts may be small, e.g. 0.25 inch diameter, and up to two inches for large, high fiber count, cables. This description will refer to these inner ducts as microducts, and to the assembly of microducts as microduct conduit.
The optical fiber medium that is installed in the microducts may comprise a single optical fiber, or small bundles typically with 2-24 optical fibers. In assemblies with, for example, 3-8 optical fibers, the fibers may be arrayed in a regular geometric pattern comprising a tape or ribbon. For high capacity systems, the optical fiber medium may comprise stacked fiber ribbons in a round or oval bundle. In all these cases the optical fiber medium should have physical characteristics that make it suitable for installation in microducts. Accordingly, in this description the assembly of optical fibers will be referred to as microduct cable.
The individual optical fibers for the microduct cable are provided with conventional primary and secondary optical fiber coatings. The assembly of optical fibers is collectively coated with an outer sheath to form the microduct cable.
In the preferred case, installation of microduct cable in microduct is achieved using air blowing techniques. Success of optical fiber air blown installations depends on several conditions such as, diameter of the microduct cable, diameter of the microduct, friction characteristics of materials of the microduct, air flow rate, air pressure, amount of vertical rise, tube obstructions, tube discontinuities, etc. Especially important in this context are the properties of the microduct cable coating, in particular the frictional characteristics of the material forming the sheath. It is desirable to have a low friction surface to allow the microduct cable to easily slide within the microduct. However, at the same time it is desirable to have sufficient roughness at the sheath surface to provide enough dynamic air drag for the air stream to convey the microduct cable through the microduct. These requirements suggest a delicate design balance for the surface friction of the sheath covering microduct cable. To first order, the friction characteristics of the tube microduct cable sheath depend on the surface roughness of the sheath. The effect of the force of the air flow on the microduct cable sheath also depends on the aerodynamic properties of the surface. In some case that surface is shaped to provide more effective movement of the microduct cable through the microduct. However, the surface roughness of the sheath is still a dominant factor. Microduct cable sheath materials are polymers, which typically give very smooth surfaces. Proposals have been made to modify the surface by adding solid particulates to the polymer. See for example, U.S. Pat. Nos. 5,533,164; 5,851,450. These have met with limited success because they tend to adversely affect other mechanical properties of the microduct cable.
Solid particulates have been added to the outer jacket of extruded optical fiber cable. See for example, U.S. Pat. No. 6,324,324. However, improved cable designs are still sought.